Blast furnaces are the primary source of high-purity iron for steelmaking. High-purity iron is required for the manufacture of the highest quality steels which must have minimal levels of detrimental elements, like copper, which are difficult to remove chemically from steel. Blast furnaces are also used in the production of other metals such as ferromanganese and lead.
Traditionally, metallurgical coke has been the primary fuel and the source of the reducing gas consumed in the blast furnace process. Coke, fluxes and ore, such as iron ore, are charged in layers at the top of the furnace, and a hot air blast is blown into the bottom of the furnace. The air reacts with the coke, generating heat for the process and producing a reducing gas which preheats the coke, fluxes and ore, and converts the iron ore to iron as it flows up through the furnace. The gas exits the top of the furnace and is used in part as a fuel to preheat the air blast.
Metallurgical coke is formed by heating coal in the absence of air, driving off the more volatile components of coal. Many of these volatile components are environmental and health hazards, and cokemaking in recent years has become increasingly regulated. The cost of complying with these regulations has raised cokemaking operating costs and increased the capital required for new cokemaking facilities. As a result, the supply of coke is shrinking and prices are rising. These factors have led blast furnace operators to decrease the amount of coke they use and to inject large amounts of alternate fossil fuels into the hot air blast supply to the furnace as a substitute. The most common fossil fuels injected are pulverized coal, granular coal, and natural gas. Pulverized and granular coal are preferred for economic reasons.
Coke is preheated by the reducing gas as the gas flows up the furnace. In contrast, the alternate fossil fuels are injected at ambient temperature. Accordingly, the addition of such fuels into the blast air supply adds a thermal load to the furnace which does not occur when only coke is used as the fuel. Operators of blast furnaces have addressed this problem by adding oxygen to the blast air and this has provided some benefit. However, even with oxygen addition, blast furnace operation at higher fossil fuel injection levels has not been achievable because of blast furnace operating problems related to poor or incomplete combustion of injected fossil fuels.
Accordingly it is an object of this invention to provide a method for providing blast air with fuel and oxygen for subsequent passage into a blast furnace which will enable improved operation of the blast furnace.